Vitreous enamel base stock, vitreous enameled articles and method



Oc 2, 1 51 A. E. CHESTER ET AL VITREOUS ENAMEL BASE STOCK, VITREOUSENAMELED ARTICLES AND METHOD Flled 5 pt 14 1949 Patented Oct. 2, 1951viTR oUs E AMEL BASE s'roox, vfrnn-oes' ENAMELED ARTICLES AND -METHODAllan E. Chester, Highland Park, and Irwin Herbert Schram, Jr.,Waukegan, 111., assignors to Poor & Company, Chicago, Ill., acorporation of Delaware Application September 14,1949, Serial No.115,706

- 12 Claims. 1 This invention relates to vitreous enameling, moreparticularly to a new and improved base stock forvitreous enamels, andto a new and improved method of controlling the adherence of vitreousenamels to ferrous metals.

It has long been recognized in the art of vitreous enameling that itwould be desirable to apply a single coat of vitreous enamel to aferrous metal sheet and thereby eliminate the necessity for a ground orgrip coat of vitreous enamel. It has also been recognized in theart thatit would be desirable to provide a ferrous metal base sheet to which lowfiring and high firing enamels would adhere and which would also havenonwarping properties and other desirable characteristics. Despite thisrecognition, the problem of providing a ferrous metal base sheet of thecharacter described has not heretofore been solved in a satisfactorymanner. Even the special low carbon enamel irons which have been provento be the best from the standpoint of less warpin with high firingtitanium oxide vitreous enamels must have a ground coat of a vitreousenamel prior to the finish coat of vitreous enamel in order to secureproper adherence and to avoid such defects as black specking.

One of the objects of the present invention is to provide a new andimproved ferrous metal base sheetto which a finish coat of vitreous fromvarious types of vitreous enamels in white and pastel colors with butone coat of enamel. Other objects will appear hereinafter.

In accomplishing these objects in accordance with this invention it hasbeen found that a new and improved ferrous metal base sheet for vitreousenamels may be prepared by a process involving (1) Surface etching theferrousmetal base stock, preferably with a fine grained etch,

. in a sulfuric acid bath which preferably also contains an oxidizingagent to accelerate the pickling action and an inhibitor to prevent theformation of iron compounds; (2) applying to the etched surface of saidbase stock, preferably after rinsing but before drying, a bathcontaining P205 in chemically combined form and a reducing sulfurcompound capable of liberating free sulfur;

(3) applying to the resultant surface of said base stock, preferablyafter rinsing or drying, a coating of antimony; and (4) applying to theantimony coating, preferably after rinsing with water or drying, acoating of nickel.

In order to accomplish the purpose of the invention and to provide aferrous metal base stock which can'be employed for a one-coat vitreousenamel of either a high firin or low firing type, it is important to usea ferrous metal, such as ingot iron or other ferrous metal, wherein themaximum carbon content does not exceed 0.05% carbon and which isessentially free from elements other than iron except for smallquantities of manganese and possibly minute quantities ofsulfur andphosphorus. The low carbon content is desirable because the probabilityof warpage after firing with high firing enamels, i. e, enamels fired attemperatures above about 1500 degrees F., increases as the carboncontent exceeds 0.05%. If the probability of warpage is not too seriousthe manganese content may be as high as 0.30%. The sulfur and phosphoruscontents should be as low as possible. The chemistry of the ferrousmetal base stock, therefore, may

the practice of the present invention with ferrous metal base sheetsconsisting essentially of. iron containing .02 to .03 carbon and .04 to.05 manganese, and with ferrous metal base sheets containing .02 to .04carbon and .12 to .15 manganese. The gauge of the sheets may vary over awide range as, for example, from 4 to 31 gauge, depending upon the usewhich is to be made of the finished vitreous enamelled article.

In the accompanying drawings I Figure 1 represents in perspective asheet of ferrous metal base stock which has been subjected on one sideonly to the four-phase treatment outlined above;

. Figure 2 illustrates the ferrous metal base stock of Figure 1 with acoating of vitreous enamel thereon; and Figures 3 and 4 illustrate aferrous metal base ,stock corresponding respectively to that shown Atypical anal-.

tons sulfate is preferably. "added to the -first pickling bath in orderto condition-the bath. The

ferrous sulfate however, tendsto build up in the bath as successivequantities of material are passed through or immersed (in the-'ibath.As,

the ferrous sulfate of the bath increases, however, the rate ofactivation of the ferrous base metalby the etching bath becomes slowerand it :is preferred for the "purpose of this invention to'maintain aferrous sulfate concentration not greater than about 2 pounds FeSOmHzOper gallon ofthe etching bath. The initial ferrous sulfate concentrationis preferably about 17.5 grams per 'gallonpfbath.

"Sodium bisulf ate 'is preferably employed in the first-bath forits'buffering action and toassistin obtaining a cleaner sheet.

It 'has beenfound that the sulfuric acid pickle alone without anyauxiliary ingredients will not v give *the desired result-due to theformation of spots or black specks after the vitreous-enamel coat hasbeen applied to the base sheet. The presence of nickel andcobalt in thelfirst pickling bath tends to aggravate black specking and 'thereforeshould-be avoided. For the same reason it isdesirable that thefirst/pickling bath 'be essentially-free from chlorides such as would beproduced, for'example, by adding hydrochloric acid to the'pickling bath;The black speckin'g seems 'to be "due inpart at least to the "formationof iron salts or other -impur-ities. The black "speck'ingdiflicultyisoVercome'in the practice of the invention Joy-adding to the firstpickling or surface etching 'bath'a salt of an alkali metal oran'alkaline'earth metal salt 'or a titanium salt.

The best results have-been obtained with stronitium salts, as, forexample, strontium carbonate. "Other specific I illustrations of saltsthat have been employed effectivelyare lithium carbonate (152603)lithium phosphate "(LiaPOiiJ/ 'HZ'O.) and titanyl sulfate(TizfSOiiaiSHzO) The temperature andtime of treatment -in"the first bathwill vary depending upon the gauge "and other physical characteristicsof the fer-rous metal being etched but in general the temperature of theetching bath will vary Within the "range of 14'0 degrees Feto 185degrees F. andthe time of treatment will vary within-a rangeof 12 to 25minutes. For a -2O gauge enamel iron-sh'eet the optimum temperature-isaround-1'55 degrees F. 'to 160 degrees .for 15 minutes infthes'firsthath. For heavier sheets the temperature and time of treatment may bemore and. .for lighter sheets the temperature and time of -treatment maybe less.

*It' preferable-toeffe'ct the treatmentzin the first surface etchingbath under asfoamwblanket which can he obtained in a well rknownimannerx by-adding a smallamount :of a foaming agent tothebath. I

Afterthe-surface etching in the first-bath it "is .preferable to rinse'the ferrous -metal sheet or to prevent formation of iron salts. Therinsin may be effected at ordinary temperatures of say degrees F. to 75.degrees F. with tap water which is preferably maintained slightly acid,that is, around a pH of 3 to 3.5 by the addition of-small amounts,usua11 yi5%"to ;1.% by weight,

of sulfuric acid. 'The acidaddition tends to prevent the formation ofiron salts.

In what has been described as the second phase ofthe invention .theferrous metal sheet or other article; which has been surface etched inthe first phase is further treated in a bath which has anadditionaletchin'g action on the sheet and provides a-coating 'o'f'whatmay be described as an 15 oxidation promoting agent.

Both sulfur and phosphorus appear to be effective for the purzposeofsthe invention. The presence of sulfur in particular seems to beessential in order to secure the-desired adherence of the subsequentlyapplied vitreous enamel. Attempts to. use other oxidizing :agentssuchas: sodium chlorate:zh'ave proved:to-beineifective.

Sulfur is supplied tothe'ferrous iba'se qnetalin the second phase of the-.imzenti'on1 by addingdzothe etching bathiemployedlforitheetreatmentinfth'e ferrous-metal iaisulfurficompoundiiwhic e- =composes to :liberateFfree :sulfur. I he sulfurrais thereby-s formed J incia nascentizstateas i colloidal sulfur. .Amongsthe compounds .whichrhavezbeen found :;to."be effectiveiaresodiumchydrosulflte and sodium xthiosulfate.v lathersulfur :compountls such asithe. polysulfides mayrberused.Therhydrosulfites :and :the thiosulfatcssboth have-:reflucing actions inan acidibathflbut'whetheraorrmit a reduction poccurs ndoes notappear toheumportant'f or the purpose of :theinyention.

'By employing. :phosphoric: acid :in attic second etching :bath:zanda-addingea; sulfur aliberating oom,-

pound thereto, ":it is epossible .ato .sproduce :asheet containing bothphosphorus-randrsulfur. "Addition .ag'ents zwlhicht'inhibit -..the=formation of salts such as those .described iwithtrespect' :to the:first phaseofthe invention shouid iaisomreferably be employed :in.thersecondphase of the invention.

The :temperaturecof :th'e etreatment in the-second phasesofithe:invention mayvaryizwithin' the range-of 14.0 adegreesv Fmto :185degreesFi, as in thedfirst phase.

' The time 10f treatment inn-the secondmhajseisof "theinventi'on mayvary, depending upon the-adesired-gphosphaterooating :on: the sheet;andiismreferably within the range of 5 to 25 minutes.

- Theq-uantity'x'ofephosphorus .011 ithesheet after the :second phase of-the--:iny.ention'isasubject to variation :but .is preferably -.:withinthe: range-10f .0005 to 0.03 gram per square foot cuff-surface,

calculated as 13205.

:After-the-:secondxphase of :the zinyentiomathe ferrous metal basersheeltf or other article im'ay. 'zbe dried directly or :may :be rrinsed with .cold: water and is thensiready.-.rfor 'themthird ephase of'izhe V "invention; I

.In the athird' rphaseiiof :the invention" the eetched surfaceof :the;ferrous :metal. is sealedwi tlaia coating-10f. antimony." 'Thissmayzbeaccomplished either :byr-plating the antimony ith applied 'elemtromotiveaforce orbyxzpla'ting'zit' iby overeyoltage out of :a suitablebath. .An :antimony'tbathr-which is'essentially antimony potassiumtartrateiis.pref- 'erable for the apurposepf :the :invention. A; reduc-'-ing 'a'gentgpreferably :a.ithiosulfata -mayybe" employe'd-assanzzoptional :ingredient-:-in'-theantimony ba'thaufhesantimonyscoatingiin"conjuncitiomwith the-su-bsequently'.appiiedcnickel acoating: improves other article-toavoid"carryover of the bath and' theadhereneerpfzthevitreousienamelp-sheet. even superimposed on direct current.

when the second phase of the invention isomitted. The thickness of theantimony coating may vary but is preferably within the range from 5 to50 millionths inch.

After the third phase of the invention the resultant product may bedried directly or rinsed with water and is then ready for theapplication of a thin plate of nickel. The nickel is preferably appliedby an over-voltage from an alkaline bath of nickel salts but may beapplied by electrodeposition using applied electromotive force either inthe form of direct current or alternating current The amount of nickelapplied to the surface of the ferrous metal base stock is subject tovariation but is preferably within the range of 7 to 20 millionths inchthickness. After the nickel coat has been applied over the antimony coatthe sheet may be dried and is then ready for vitreous enameling witheither low firing or high firing types of vitreous enamels. Excellentresults have been obtained in the application to this type of sheet of asingle coat of a high firing titanium oxide enamel which is fired at1480 degrees F. to 1540 degrees F.

The invention will be illustrated, but is not lim- .ited by thefollowing examples in which the quantities are stated in parts by weightunless otherwise indicated.

Example I Sheets of iron in the form of racks consisting of 6 sheetseach of a 20-gauge thickness and 3" x 6" dimensions are etched in a bathcomposed of the following ingredients:

280 cc. sulfuric acid (concentration suificient to give 6 H2804 byweight of bath) 35 grams ferrous sulfate (FBSO4.7H2O) 10 grams sodiumbisulfate (NaHSO4.H2O)

2 grams lithium phosphate (Li3PO4. H2O) 1 gram manganese dioxide 0.5gram isopropylnaphthalene sodium sulfonate (Santomerse No. 1)

200 cc. of 72% phosphoric acid fHaPOr) 2 grams lithium phosphate(LisPorJ/zHzO) 1.14 grams sodium hydrosulfite (Na2S2O4) made up to 2gallons with water.

The sheets are immersed in this bath at a temperature of 155 degrees F.to 160 degrees F. for minutes. They are then removed from the bath andrinsed with cold water at a temperature of 60 degrees F. to 75 degreesF.

The treated sheets are then immersed in a bath composed of the followingingredients:

76 grams tartar emetic imsboicmioa /mzoi 152 grams sodium thiosulfate(NazSzOafiHaO) made up to 2 gallons with Water.

The pH of this bath is held around 5.5 to 5.7 and the sheets areimmersed for a period of time from /2 minute to 15 minutes.

The antimony plated sheets are rinsed with "water at 60 degrees. F. to75 degrees F; and are;

immersed in an alkaline nickel plating bathhaving the followingcomposition: I

made up to 2 gallons with water and to a pH of 8 to 10 with diluteammonium hydroxide.

The nickel is plated onto the antimony coating of the ferrous metal basesheet by over-voltage at a temperature of 190 degrees F. to 200 degreesF. for periods varying from 1 minute to 15 minutes. I

In preparing the foregoing bath the nickel chloride and ammoniumchloride are mixed together and purified by the addition of 10 cc. ofhydrogen peroxide at a temperature of degrees F. The hydrogen peroxideoxidizes any iron present and the iron precipitates when the pH israised to 8 to 10 by the addition of the ammonium hydroxide. Diluteammonium hydroxide is employed because concentrated ammonium hydroxidewill precipitate nickel. The sodium hypophosphite in this bath acts as areducing agent and-the quantity thereof may vary within the range of 0.5gram to 15 grams per liter of bath.

The resultant ferrous metal base stock when coated with a vitreousenamel and fired yields an excellent vitreous enamel coated sheet. Forexample, a high firing titanium oxide enamel (Chicago vitreous enamelCV-1452) was applied to sheets prepared as above described and fired at1520 degrees F. for 3 to 4 minutes. The resultant sheet showed excellentadherence and freedom from black specking.

Example II 1 200 cc. of 72% phosphoric acid (H3PO4) made up to 2 gallonswith water.

An analysis of the sheetafter various time intervals in the foregoingbath showed the following P205 content on the surface:

The process is carried out as described in Example I except that thesodium thiosulfate is omitted in the antimony potassium tartrate bath(tartar emetic bath). Good results are obtained but not quite as good asthose obtained with the s ium thiosulfate.

Example IV The invention is carried out as described in Ex.- ample Iusing sodium hydrosulfite instead of sodium thiosulfate in the antimonyplating bath in the same proportions with substantially the sameresults. I v

. Ex'ample V v The invention is""carried out a'sdesc'ribed m Example I'using a 'nickel'bath of the-followin composition-instead of thealkaline. nickel bath described in Example- I:

2272-grams nickel' 'sulfate (NiSOefiH-zO) 4 54.grams nickel chloride(NiClz) I14 grams citric acid made'up toz gallonsiiwith water.

This bath is purified --in a mannersimilar ?to the a1ka1ine" bath bymixing *the nickel sulfate and nickel chloride with water at atemperature 01 160 degrees F., bringing the pH to 6, adding hydrogenperoxide E'and ifiltering. The citric acid *is added to the filtrate andthe ,pH is brought to-'z=2 .'5 by adding nickel carbonate. -Nick'elisplated out ofethe bathonto'the ferrous metal 'base'stock at atemperature of '140: degrates-F. to 160 degrees F. at 4oamperespersquare foot current density. Various thicknessesof the nickel-are platedby setting the rheostatat :10 amperes'and-varying theplating time overa. perlodof 5 to 50 seconds.

Thecitric' acid in this bath functions primarily asea bufier. Tartaricacidmay be .used instead of citricaoidbut is not as effective. Likewise.other buffers, such as boric acid,are'not as effective as citric acid.

It will be .understood that variations maybe made in the-exact procedureandipropo'rtions describedabove provided they do "notdepart fromithe-invention. If the baths are to be usedaover and 'over a'gain theingredients thereof must be ;-repleni'sh'ed 'from time to time. *Forexample; in the phosphoric acid bath described ins-Example I the sodiumthiosulfate may be replenished by adding .57 gram of sodiumthiosulfatefor each rack of six sheets 3".x 6. Those-skilled inthe art can readilydetermine .by .simpletestswhat additions are necessary toreplenishargiven bath for treating ferrous metal .base stock.

'Referring' to the drawings and the. foregoing examples it will be seenthat in Figure lthe ferrous metal base stock comprises a sheet I havingasurface etch'shownas alayer '2 which 1 corresponcls-to the-first phasetreatmentya phosphorus and sulfur containing coating 3 corresponding tothe second phase treatment, an antimony *coating 'l'corresponding tothe'third phase treatment and a" nickel coating-5 correspondingto'the-fourth phase treatment.

a Figure 2 illustrates the ferrous metal base stock shown in Figure 1after the application of a vitreousenamel coat 6.

In Figures 3 and 4 the coatings on the ferrous metal base stockscorrespond to those in Fig- -'ures 1 and 2 and are given like numeralsexcept that the coatings are on both sides of'the sheet.

It is believed that the invention provides for the first timea"'newa-andimproved type of ferrous metal base stock which issatisfactory "for the appli'catio'n of a-singl'e' finish coat of "eitherwhig'h firing 'or' low firingv-itreous enamel without the necessity-fora ground coat. invention-also provides ferrous metal b'ase'stocks-char:a'.cterizedby improved adherence to vitreous enamels. Additionallythe invention provides ferrous metal base stocks which may be enamelleddirectly with White or pastel shades of -vitreous enamels without blackstreaking.- The "vitreous enamelled--basestocks --an"dthe vitreouse-na'melled articles provided in :accordance with the 'inventionare-a-lso essentially freefrom warpage and other undesirablecharacteristics.

'The 1:

ing 4%.to 8% by weight sulfuricacid,.aquantity The invention amakes it:possible-for .the enamelertio'employ zany of the commercially availablevitreous enamel fritswithout'a special-knowledge as to the exact natureof the fritand-without applying aeground coat to-the base coat.

The invention'ishereby claimed as-follows: .1. :In ta .method ofpreparing low carbon Eferrous metals Ifor vitreous enameling, theystepswhich consist essentially in acid picklingsaid ferrous .metal' i in 2asulfuric acid 1 bath, depositing :on the pickled surface 'a coating of aph sphate and elementalsulfur,:sealing said deposited coating with athin :coating of antimony :and sealing .said coating of .antimony with a:thin coating-of nickel.

2. A method of :controlling the adherence aof vitreous enamels stolowcarbon ferrous-metals in a sulfuric acid bath :which comprises 3-68-sentially 'acid pickling said ferrous metal,a:depositing on the pickledsurface a coating :of-xa phosphate and elemental sulfur,:sealing saidzdeposited coating with a thin coating-of antimony, sealing said coating ofantimony with athin coating of nickel and'firing a vitreousenamel oversaid nickel coating.

--3. -In a method of: preparing ferrous metalsifor vitreous enamelingpthe steps which comprise .es-

depositing on the pickled surface-a coating. of-a phosphate andelementalsulfur, sealing said-deposited coating with a'thin coating of antimonyand sealing saidccoatingofantimony with'athin coatingof nickel.

4. A method of controlling:the adherence-of vitreous enamels toferrous-metals which comprises essentially acid pickling in asulfuricacid bath said ferrous metal .havingan average ,carbon contentnotgreater than about 0.05 by weight, an average manganese content notgreat:

er than 0.30% by weight, an average phosphorus content not greater thanabout 0.010% by weight, an average sulfur content not greater than about0.025% by-wei'ght and the remainder beinges sentially iron, depositingon..the pickled surface a coating of .a phosphate and elemental sulfur,

sealing said deposited coating with athincoating of antimony, sealingsaid coating of antimony with a thin coating of nickel and firing avitreous enamel over said nickelcoating.

5. In a -:methodof preparing a low carbon ferrous metal for vitreousenameling, the s: steps which comprise essentially pickling a ferrousmetal in a sulfuricacid picklingibathcontainof an oxidizing'compound ofmanganeseeffectiveto accelerate the rate of picklingand .a

elemental sulfurinsaid bath: to form;a' ferrous metal having anetchedasurfaceiwith: a coatipg ing sa'id coating with..a .thin: plated.icoatingnof 9 antimony and sealing said antimony coating with a thinplated coating of nickel.

6. A method of controlling the adherence of vitreous enamels to lowcarbon ferrous metals which consists essentially in pickling a ferrousmetal in a sulfuric acid pickling bath containing 4% to 8% by weightsulfuric acid, a quantity of an oxidizing compound of manganeseeffective to accelerate the rate of pickling and a quantity of aninhibitor for the formation of black specks in the enamelled productsrinsing the resultant pickled ferrous metal in a slightly acid bath,treating the pickled surface of the ferrous metal with a phosphoric acidetching bath containing an inhibitor for the formation of black specksin the enamelled product and a compound which decomposes to liberateelemental sulfur in said bath to form a ferrous metal having an etchedsurface with a coating of phosphate and elemental sulfur thereon,sealing said coating with a thin plated coating of antimony, sealingsaid antimony coating with a thin plated coating of nickel and firing avitreous enamel over said nickel coating.

7. In a method of preparing a ferrous metal for vitreous enameling, thesteps which comp-rise essentially acid pickling a ferrous metal havingan average carbon content not greater than about 0.05% by weight, anaverage manganese content not greater than 0.30% by weight, an averagephosphorus content not greater than about 0.010% by weight, an averagesulfur content not greater than about 0.025% by weight and the remainderbeing essentially iron in a sulfuric acid bath containing 4% to 8% byweight sulfuric acid, a quantity of ferrous sulfate between 17.5 gramsand two pounds per gallon of bath, a quantity of manganese dioxide, anda quantity of lithium phosphate, rinsing said pickled ferrous metal witha solution of 0.05%, to 1% sulfuric acid, treating the pickled surfacewith a bath of dilute phosphoric acid containing lithium phosphate and aquantity of an inorganic reducing sulfur compound decomposable in thebath to deposit elemental sulfur, sealing the resultant coated surfaceof the ferrous metal with a thin coating of antimony in the presence ofa reducing inorganic sulfur compound which decomposes to liberateelemental sulfur and sealing the antimony coating with a thin coating ofnickel.

8. A method of controlling the adherence of vitreous enamels to ferrousmetals which consists essentially in acid pickling a ferrous metalhaving an average carbon content not greater than about 0.05% by weight,an average manganese content not greater than 0.30% by weight, anaverage phosphorus content not greater than about 0.010% by weight, anaverage sulfur content not greater than about 0.025% by weight and theremainder being essentially iron in a sulfuric acid bath containing 4%to 8% by weight sulfuric acid, a quantity of ferrous sulfate between17.5 grams and two pounds per gallon of bath, a quantity of manganesedioxide, and a quantity of lithium phosphate, rinsing said pickledferrous metal with a solution of 0.05% to 1% sulfuric acid, treating thepickled surface with a bath of dilute phosphoric acid containing lithiumphosphate and a quantity of an inorganic reducing sulfur compounddecomposable in the bath to deposit elemental sulfur, sealing theresultant coated surface of the ferrous metal with a thin coating ofantimony in the presence of a reducing inorganic sulfur compound whichdecomposes to liberate elemental sulfur, sealing the antimony coatingwith a thin coating of nickel and firing a vitreous enamel over said,nickel coating.

9. A vitreous enamel base stock consisting essentially of a sulfuricacid pickled low carbon ferrous metal having on the pickled surfacethereof a coating of phosphate and elemental sulfur, a thin coating ofantimony over the phosphate and sulfur coating and a thin coating ofnickel over the antimony coating.

10. A vitreous enamel base stock consisting essentially of a sulfuricacid pickled ferrous metal sheet having an average carbon content notgreater than 0.05% by weight, an average manganese content not greaterthan 0.30% by weight, an average phosphorus content not greater than0.010% by weight, an average sulfur content not greater than 0.025% byweight and the remainder being essentially iron, a coating of phosphateand elemental sulfur on the pickled surface of said sheet, a thincoating of antimony over said phosphate and said sulfur coating and athin coating of nickel over said antimony coating.

11. A vitreous enamelled articleobtained by firing a vitreous enamel ona low carbon ferrous metal which has been acid pickled in a sulfuricacid bath and has had a coating of phosphate and sulfur deposited on theacid pickled surface and sealed with successive coatings of antimony andnickel.

12, A vitreous enamelled article obtained by firing a vitreous enamel ona ferrous metal having an average carbon content not greater than 0.05%by weight, an average manganese content not greater than 0.30% byweight, an average phosphorus content not greater than 0.010% by weight,an average sulfur content not greater than 0.025% by weight and theremainder being essentially iron, which has been acid pickled in asulfuric acid bath and has had a coating of phosphate and sulfurdeposited on the acid pickled surface and sealed with successivecoatings of antimony and nickel.

ALLAN E. CHESTER. IRWIN HERBERT SCHRAM, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,207,218 Roux Dec. 5, 19162,121,520 Curtin June 21, 1938 2,271,706 Morris Feb. 3, 1942 FOREIGNPATENTS Number Country Date 507,355 Great Britain June 14, 1939 OTHERREFERENCES Lange: Handbook of Chemistry, 6th ed.,

pages 196, 197 and 1169, publ. 1946 by Handbook Publishers, Inc.,Sandusky, Ohio.

6. A METHOD OF CONTROLLING THE ADHERENCE OF VITREOUS ENAMELS TO LOW CARBON FERROUS METALS WHICH CONSISTS ESSENTIALLY IN PICKLING A FERROUS METAL IN A SULFURIC ACID PICKLING BATH CONTAINING 4% TO 8% BY WEIGHT SULFURIC ACID, A QUANTITY OF AN OXIDIZING COMPOUND OF MANGANESE EFFECTIVE TO ACCELERATE THE RATE OF PICKLING AND A QUANTITY OF AN INHIBITOR FOR THE FORMATION OF BLACK SPECKS IN THE ENAMELLED PRODUCTS RINSING THE RESULTANT PICKLED FERROUS METAL IN A SLIGHTLY ACID BATH, TREATING THE PICKLED SURFACE OF THE FERROUS METAL WITH A PHOSPHORIC AICD ETCHING BATH CONTAINING AN INHIBITOR FOR THE FORMATION OF BLACK SPECKS IN THE ENAMELLED PRODUCT AND A COMPOUND WHICH DECOMPOSES TO LIBERATE ELEMENTAL SULFUR IN SAID BATH TO FORM A FERROUS METAL HAVING AN ETCHED SURFACE WITH A COATING OF PHOSPHATE AND ELEMENTAL SULFUR THEREON, SEALING SAID COATING WITH A THIN PLATED COATING OF ANTIMONY, SEALING SAID ANTIMONY COATING WITH A THIN PLATED COATING OF NICKEL AND FIRING A VITREOUS ENAMEL OVER SAID NICKEL COATING. 